The present invention relates to improved techniques for catalytic reforming, particularly, catalytic reforming under low-sulfur conditions. More specifically, the invention relates to the discovery and control of problems particularly acute with low-sulfur reforming processes.
Catalytic reforming is well known in the petroleum industry and involves the treatment of naphtha fractions to improve octane rating by the production of aromatics. The more important hydrocarbon reactions which occur during the reforming operation include the dehydrogenation of cyclohexanes to aromatics, dehydroisomerization of alkylcyclopentanes to aromatics, and dehydrocyclization of acyclic hydrocarbons to aromatics. A number of other reactions also occur, including the dealkylation of alkylbenzenes, isomerization of paraffins, and hydrocracking reactions which produce light gaseous hydrocarbons, e.g., methane, ethane, propane and butane. It is important to minimize hydrocracking reactions during reforming as they decrease the yield of gasoline boiling products and hydrogen.
Because there is a demand for high octane gasoline, extensive research has been devoted to the development of improved reforming catalysts and catalytic reforming processes. Catalysts for successful reforming processes must possess good selectivity. That is, they should be effective for producing high yields of liquid products in the gasoline boiling range containing large concentrations of high octane number aromatic hydrocarbons. Likewise, there should be a low yield of light gaseous hydrocarbons. The catalysts should possess good activity to minimize excessively high temperatures for producing a certain quality of products. It is also necessary for the catalysts to either possess good stability in order that the activity and selectivity characteristics can be retained during prolonged periods of operation; or be sufficiently regenerable to allow frequent regeneration without loss of performance.
Catalytic reforming is also an important process for the chemical industry. There is an increasingly larger demand for aromatic hydrocarbons for use in the manufacture of various chemical products such as synthetic fibers, insecticides, adhesives, detergents, plastics, synthetic rubbers, pharmaceutical products, high octane gasoline, perfumes, drying oils, ion-exchange resins, and various other products well known to those skilled in the art.
An important technological advance in catalytic reforming has recently emerged which involves the use of large-pore zeolite catalysts. These catalysts are further characterized by the presence of an alkali or alkaline earth metal and are charged with one or more Group VIII metals. This type of catalyst has been found to advantageously provide higher selectivity and longer catalytic life than those previously used.
Having discovered selective catalysts with acceptable cycle lives, successful commercialization seemed inevitable. Unfortunately, it was subsequently discovered that the highly selective, large pore zeolite catalysts containing a Group VIII metal were unusually susceptible to sulfur poisoning. See U.S. Pat. No. 4,456,527.
Generally, sulfur occurs in petroleum and syncrude stocks as hydrogen sulfide, organic sulfides, organic disulfides, mercaptans, also known as thiols, and aromatic ring compounds such as thiophene, benzothiophene and related compounds.
Conventionally, feeds with substantial amounts of sulfur, for example, those with more than 10 ppm sulfur, have been hydrotreated with conventional catalysts under conventional conditions, thereby changing the form of most of the sulfur in the feed to hydrogen sulfide. Then, the hydrogen sulfide is removed by distillation, stripping or related techniques.
One conventional method for removing residual hydrogen sulfide and mercaptan sulfur is the use of sulfur sorbents. See, for example, U.S. Pat. Nos. 4,204,997 and 4,163,706, the contents of which are hereby incorporated by reference. The concentration of sulfur in this form can be reduced to considerably less than 1 ppm by using the appropriate sorbents and conditions, but it has been found to be difficult to remove sulfur to less than 0.1 ppm or to remove residual thiophene sulfur. See, for example, U.S. Pat. No. 4,179,361 the contents of which is hereby incorporated by reference, and particularly Example 1 of that patent. Very low space velocities are required to remove thiophene sulfur, requiring large reaction vessels filled with sorbent. Even with these precautions, traces of thiophene sulfur still can be found.
Thus, improved methods for removing residual sulfur, and in particular, thiophene sulfur, from a hydrotreated naphtha feedstock were developed. See, for example, U.S. Pat. Nos. 4,741,819 and 4,925,549, the contents of which are hereby incorporated by reference. These alternative methods include contacting the naphtha feedstock with molecular hydrogen under reforming conditions in the presence of a less sulfur sensitive reforming catalyst, thereby converting trace sulfur compounds to H.sub.2 S, and forming a first effluent. The second effluent is contacted with a highly selective reforming catalyst under severe reforming conditions. Accordingly, when using the highly sulfur sensitive catalysts, those skilled in the art go to great extremes to remove sulfur from the hydrocarbon feed. By doing so, the catalyst life is extended for significant periods of time.
While low-sulfur systems using highly selective large-pore zeolite catalysts were initially effective, it was discovered that a shut down of the reactor system may be necessary after only a matter of weeks. The reactor system of one test plant had regularly become plugged after only such brief operating periods. The plugs were found to be those associated with coking. However, although coking within catalyst particles is a common problem in hydrocarbon processing, the extent and rate of coke plug formation far exceeded any expectation.